A personal answer from Gerald.
The role of prophet is to call people to repentance and to have faith in Christ. Part of this responsibility is to occasionally receive new revelation from God and transmit the new commandments and teachings to the people.
Each prophet tends to have a specific focus. Noah was warned about the Flood. Moses gave the Law. Isaiah was called to command the nations to repent. Ezekiel explained to Israel that Jehovah was still their God, even in their Babylonian exile. Each prophet tends to have a special dispensation of specific responsibilities in leading and guiding the people of their time and place.
Today is no different. Joseph Smith needed to restore many precious lost things, and so received many revelations. Brigham Young had to lead the Saints west and tame the western deserts. Today, Thomas S. Monson receives inspiration to lead an international church, to see to it that all members have access to a temple, and to prepare for difficult times to come. Each prophetic dispensation focuses on the current and future needs of the Church and people, often receiving greater enlightenment on previous revelation (see Doctrine and Covenants 138 for an example).
All prophets and apostles walk by faith (2 Corinthians 5:7) just like we are required to do. With Paul, they sometimes see “through a glass darkly” (1 Corinthians 13:12). God reveals knowledge to them when they need it and seek it.
Prophets are human and fallible. Prophets do not know everything. They have not had all knowledge revealed to them, but only the portion God chooses to share with them.
Occasionally, God will make changes to the structure and instructions of the religion. Prior to Peter receiving the command to take the gospel to Cornelius, only Jews could be taught. Jesus fulfilled and superseded the Law of Moses, which was called an “everlasting covenant” by Moses.
On occasion, prophets and apostles will disagree, due to personal perceptions. Two people can see the same event and perceive it somewhat differently. In explaining the Vision of the Tree of Life, Nephi (a Book of Mormon prophet) said his father, Lehi, did not notice that the river was filthy because he was focused on other things (1 Nephi 15:27).
Prophets always agree on the core principles and precepts that have been clearly revealed by God. As with the rest of us, they seek to understand more, using logic and reason to understand those things not yet revealed by God.
Occasionally this will cause the prophets and apostles to disagree.
For example, our leaders are often politically diverse. Politically, some focus on concepts of personal responsibility and freedom, and will choose a party that agrees. Others may focus on the need to care for the poor and suffering, and choose a different political party to follow. However, when it comes down to the key principles and core doctrines, they stand firmly united.
It is important to focus on what unites us, not what divides us. The early Christian leaders were often divided, with Paul usually in the middle of the controversy (see Galatians 2:11). Such division hurt the ancient Church and can hurt the Church today if allowed to happen. We see a strongly united First Presidency and Twelve Apostles today, because they focus on the doctrines that unite, and not on their personal views that could divide. Jesus taught, “If ye are not one, ye are not mine” (Doctrine and Covenants 38:27, John 17:21), and the living apostles strive to be one as Jesus commanded.